Ion thrustor cathode



y 27, 1959 P. D. READER ETAL 3,447,015

ION THRUSTOR CATHODE Filed Feb. 24, 1967 INVENTORS WILLIAM R. KERSLAKE HAROLD R. KAUFMAN PAUL D READER AFYQRNEYS United States Patent U.S. Cl. 313--231 12 Claims ABSTRACT OF THE DISCLOSURE A cathode for an electron bombardment ion thrustor in which a block of barium carbonate contacts a heated tungsten screen. The barium carbonate is reduced when heated to activate the electron emitting surface thereby lowering the temperature at which electrons are emitted. A spring-loaded piston presses the blocks against the screen so that the activator coating is replenished as the electron emitting surface is eroded by ions.

Origin of the invention The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.

Background of the invention This invention is directed to an improved cathode for an electron bombardment ion thrustor. The invention is concerned with providing a cathode having a long life while operating in such a thrustor.

Low work function cathodes utilizing thin films of barium oxide emissive coatings have been proposed for use in electron bombardment ion thrustors. These cathodes have very short lives of only a few hours in the ion chamber 'of a mercury electron-bombardment ion thrustor. It has been suggested that cathode life could be lengthened by utilizing a thick oxide built up of wires or other matrix materials. This has improved the duration of the cathodes in the thrustors considerably, but they have posed increased demands on the heaters. Also, these cathodes have undesirably long thermal time constants or lags because of the thickness of the oxide which is required to produce the long life.

Objects and summary of the invention The problems encountered with prior art cathodes have been solved by a cathode constructed in accordance with the present invention. In this cathode barium carbonate is pressed against a heated fine-mesh tungsten screen. This heat reduces the carbonate which covers the screen and activates the electron emitting surface. The carbonate has been mixed with activator and hydrostatically pressed into blocks. These blocks are placed in a duct with the heated screen at one end and a spring-loaded piston at the other which keeps the blocks pressed against the screen.

It is, therefore, an object of the present invention to provide an ion thrustor with an improved cathode that has a low heating power, an extremely low thermal time constant and constant heater characteristics.

Another object of the invention is to provide a cathode having a substantially unlimited supply of emissive material together with mechanical simplicity and rugged design.

Still another object of the invention is to provide a cathode having an emissive surface which can be reconditionedafter exposure to the atmosphere.

These and other objects of the invention will be appar- 3,447,015 Patented May 27, 1969 ent from the specification which follows and from the drawing wherein like numerals are used throughout to identify like parts.

Brief description of the drawing The drawing is a perspective view in quarter section of a portion of an ion thrustor having an improved cathode constructed in accordance with the invention mounted therein.

Descripition of the preferred embodiment Referring now to the drawing, there is shown a portion of electron bombardment ion thrustor of the type described in U.S. Patent No. 3,156,090. The thrustor utilizes a propellant, such as liquid mercury, which is supplied under pressure to a vaporizer 10. The mercury vapor flows through a tube 12 into a manifold 14 at the upstream end of the ion thrustor. From this manifold the vapor flows into an ion chamber 16 formed by a tubular casing 18 having a distributor plate 20 mounted on the upstream end. A cathode 22 is mounted on the distributor 20, and a cylindrical anode 24 is carried by the casing 18. Electrons emitted from the cathode 22 bombard the atoms of propellant vapor ionizing some of them.

The discharge is contained by an approximately axial magnetic field produced by a magnet which encircles the ion chamber 16 about the casing 18. This magnet may be of the type shown in U.S. Patent No. 3,238,715. Also, the propellant may enter the chamber in the manner shown in the U.S. Patent No. 3,262,262.

Most of the ions that diffuse at the downstream end of the chamber 16 are focused and accelerated into an exhaust beam by a potential applied to a screen grid and an accelerator grid mounted on the end of the casing 18 remote from the distributor 20. This ion beam is neutralized by electrons emitted from a neutralizer in the beam. This acceleration and focusing is described in U.S. Patent No. 3,156,090.

Oxide coated cathodes have been proposed for use in a mercury electron-bombardment ion thrustor of the type shown in the drawing. The active emitting surface of an oxide cathode is continually bombarded by ions from the discharge which causes sputtering erosion on the active surface. An attempted solution to this problem has been to provide either an increased depth of oxide or more surface area or both. The increased thickness of the oxide layer causes thermal and electrical conduction problems. Very careful design is required to avoid hot spot emission and the destruction of the oxide surface. Thermal conduction problems lead to long emission time response with respect to cathode heater power changes and can result in control problems. Increased surface area requires ingenuity in arranging the surfaces to give both acceptable ion chamber performance and a compact design which can be heat shielded.

The oxide magazine cathode 22 shown in the drawing is constructed in accordance with the invention and incorporates a large quantity of active material with a minimum coverage of the heater element with oxide. The cathode 22 utilizes a directly heated tungsten screen 30 which functions as a substrate for the active emitting material. A hollow rectangular housing 32 having a pair of outwardly directed flanges 33 on the upstream end and similar flanges 34 on the other end protrude from the forward end of the thrustor. The housing 32 is stainless steel and forms a duct for properly positioning blocks 35 of emissive material which are free to slide therein.

Each block 35 comprises a mixture of barium carbonate, carbon and stearic acid which has been hydrostatically pressed to the proper size and configuration. Each block 35 preferably includes carbon in the range from about 5 to 10 percent by weight and about 1 to 4 percent by weight of stearic acid. A generally tubular retainer 36 is removably bolted to the flange on the forward end of the housing 32 remote from the distributor 20. A stainless steel piston 38 is free to slide along the axis of the retainer 36, and a rectangular plate 40 mounted on one end of the piston 38 is free to slide in the housing 32. A tungsten wire spring 42 encircles the piston 38 and presses against the face of the plate 40 to urge the plate and piston assembly toward the heated screen 30. In this manner, the blocks 35 are sequentially pressed against the heated screen 30.

By way of example, a screen 30 was woven from 3.7 mil tungsten wire with a x 120 per centimeter mesh. The screen 30 was fabricated so that the heater current passed through it in the 10 mesh direction thereby minimizing the required heating current. The screen 30 is mounted on an insulator 44 by shaping it to the proper configuration and clamping both ends.

The insulator 44 is of boron nitride or aluminum oxide and has a pair of spaced arms 46 and 48 which extend from the distributor into the ion chamber 16. The arms 46 and 48 support the screen 30 and position the adjacent blocks 35. A pair of rigid cross bars 50 and 52 connect and position the arms 46 and 48 at the upstream end. The arms 46 and 48 and cross bars 50 and 52 are preferably fabricated from a single piece of insulating material. This insulator configuration is utilized to provide a large emitter area and to minimize thermal conduction leakages.

The distributor plate 20 has a centrally disposed circular opening covered by a mounting plate 54 that is secured to the distributor plate about the periphery of this central opening. The mounting plate 54 has a generally retangular center opening with a flange 56 extending about its periphery. The flange 56 protrudes toward the chamber 16 and engages the outwardly facing surfaces of the arms 46 and 48 of the insulator 44 as well as the outwardly directed surfaces of the cross braces 50 and 52. The downstream flanges 34 on the housing 32 are secured to the forward face of the insulator 44 which extends through the rectangular opening in the mounting plate 54.

The screen 30 is first shaped to a generally U-shaped configuration to cover the three open sides of the space between the insulator arms 46 and 48. A pair of spaced pins 58 extending between the arms 46 and 48 support the downstream portion of the screen 30.

The upstream ends of the screen 30 are clamped in the manner shown in the drawing. The opposite ends of the screen 30 pass around copper contact blocks 60 and 62 adjacent the insulator cross bars 50 and 52, respectively. Steel clamping bars 64 and 66 press the ends of the screen 30 tightly against the contact blocks 60 and 62, respectively. A pair of mounting screws 68 pass through opposed ends of the clamping bar 64, contact block 60 and insulator cross bar 50 to tightly hold the end of the screen 30. These screws are threaded into a copper bus bar 70 which engages the forward face of the insulator 44. Similar screws hold the other end of the screen 30 by passing through the clamping bar 66, contact block 62 and insulator cross bar 52 into another bus bar on the opposite side of the housing 32.

Each copper bus bar is spaced from the walls of the housing 32 to prevent shorting through this structure. The bus bars extend from the ion chamber 16 on the upstream side of the distributor plate 20. An electrical lead 72 is connected to the bus bar 70 while a similar lead is likewise connected to the opposite bus bar. An electric current passes from the lead 72 to the bus bar 70 and then through the screw 68 to the contact block 60 and clamping bar 64. This cur-rent then passes through the screen 30 to the contact block 62 and clamping bar 66 where it flows through the mounting screws to the opposite bus bar and lead.

An important feature of the invention is that the barium carbonate and carbon mixture is hydrostatically pressed into cubes or pellets 35 shown in the drawing so that the spring-loaded piston 38 can push it through the duct formed by the housing 32. When loose powder was placed in the housing 32 the piston 38 and plate 40 jammed.

It is also important that the blocks 35 contain carbon which acts as an activator. When only carbonate was used in the housing 32 only a few milliamperes of emission was obtained. With the addition of carbon activator, several amperes of emission were obtained before an emission limit was reached.

Two cathodes were constructed in accordance with the invention and tested in ion engines. Both cathodes had 11.3 square inches of emission area. The barium oarbonate blocks 35 contained 7.8 percent carbon activator and were pressed at a pressure of 50,000 psi. One of these cathodes was operated for 3240 hours while the other cathode was operated for 4179 hours.

While a preferred embodiment of the oxide magazine cathode has been illustrated and described, it will be appreciated that various structural modifications may be made to this cathode without departing from the spirit of the invention or the scope of the subjoined claims.

What is claimed is:

1. In an electron-bombardment ion thrustor having a cathode for supplying electrons to ionize a mercury propellant in a vacuum environment wherein the mercury ions erode the electron emitting susface of said cathode, an improved cathode comprising a pervious tungsten member for emitting said electrons from a surface thereof in said thrustor,

a supply of barium carbonate in contact with said electron emitting surface,

means for heating said pervious tungsten member to reduce the adjacent barium carbonate whereby said surface is activated with a coating thereof to lower the electron emitting temperature, and

means for moving additional barium carbonate from said supply into contact with said heated electron emitting surface as said activator coating is eroded away by said mercury ions.

2. A cathode as claimed in claim 1 including means for passing an electrical current through the tungsten memher for resistively heating the same to said electron emitting temperature.

3. A cathode as claimed in claim 1 wherein the pervious member comprises a tungsten screen.

4. A cathode as claimed in claim 3 wherein the tungsten screen comprises wires with a 10 x per centimeter mesh.

5. A cathode as claimed in claim 4 wherein the tungsten screen is mounted so that the current passes through in the 10 mesh direction thereby minimizing the required heating current.

6. A cathode as claimed in claim 1 including an activator mixed with the barium carbonate supply.

7. A cathode as claimed in claim 6 wherein the activator comprises carbon.

8. A cathode as claimed in claim 7 wherein the barium carbonate contains about 5 to 10 percent by weight of the carbon activator.

9. An ion thrustor cathode for supplying electrons to ionize a gaseous propellant by electron bombardment in a vacuum environment wherein the ions erode the electron emitting surface, said cathode comprising an elongated tungsten screen member for emitting said electrons from a surface thereof in said thrustor,

at least one substantially solid block of barium carbonate in contact with said electron emitting surface, means for heating said elongated tungsten screen memher to reduce the adjacent barium carbonate whereby said surface :is activated with a coating thereof to lower the electron emitting temperature, and resilient means for applying a force on said barium carbonate block towards said heated screen thereby maintaining said block in contact with said electron emitting surface as said activator coating is eroded away by said ions.

10. A cathode as claimed in claim 9 wherein about 1 to 4 percent by weight of stearic acid is mixed with the barium carbonate prior to pressing into the sol-id block.

11. A cathode as claimed in claim 9 including a hollow housing extending away from said heated tungsten member for movably holding the barium carbonate block, and

resilient means mounted in said housing remote from said heated tungsten member for moving said barium carbonate block toward the electron emitting surface.

12. A cathode as claimed in claim 11 including a piston movably mounted in the hollow housing for engaging a surface of the barium carbonate block remote from the electron emitting surface, and

6 a spring for moving said piston toward said heated tungsten member.

References Cited UNITED STATES PATENTS 2,107,945 2/1938 Hull et a1 313212 X 2,071,973 2/1937 Francis 313--212 X 2,306,290 12/ 1942] Widell 313212 10 JAMES W. LAWRENCE, Primary Examiner.

R. F. HOSSFELD, Assistant Examiner.

US. Cl. X.R. 

